Sequential batch manufacture of tetramethyl lead



April 22, 1969 W. W. RICH SEQUENTIAL BATCH MANUFACTURE OF TETRAMETHYL LEAD Filed April 19, 1957 5 I0 4 -31 6 *Q I M C/F p I j I?! 11- l9 5 l2 as 1 1 L 1! ag2 20 INVENTOR WILLIAM W. RlCH ATTORNEY United States Patent O US. Cl. 260437 Claims ABSTRACT OF THE DISCLOSURE Process for increasing the efficiency of tetramethyl lead production which comprises operating a group of methylation autoclaves singly in sequence on a fixed time schedule which includes venting only one autoclave to the low pressure bleed line at any one time. Yield and autoclave elficiency are increased.

BACKGROUND OF THE INVENTION Field of the invention This invention concerns an improvement in the batch manufacture of tetramethyl lead. More specifically, it is directed to improving the productivity of a group of tetramethyl lead autoclaves by sequential operation of the autoclaves.

Description of the prior art Tetramethyl lead is produced by a batch autoclave process. The process involves charging an autoclave with a catalyst, inert diluent, sodium-lead alloy, an autoclave lubricant and methyl chloride; heating-up and reacting the charge; venting excess pressure in two stages through a high pressure venting line and then a low pressure venting line; and discharging the reacted mixture,

Heretofore, a group of autoclaves were operated in pairs on a staggered schedule such that the entire procedure was a continuing twenty-four hour batch operation. That is, two autoclaves were charged, heated-up, etc. substantially simultaneously. After the first two were charged, the sequence was initiated on the second pair. By this means, the entire group of autoclaves was theoretically constantly employed in some stage of the sequence.

However, due to various interval and inherent difiiculties in the procedure, the system was not run at maximum efficiency. For example, manpower requirements necessitated that one autoclave of any one pair be charged before the charging of the second one of the pair was begun. Thus the pair were not operating exactly simultaneously, and the time taken to charge the first autoclave was wasted in determining the efficiency of the pair. Moreover, by operating substantially simultaneously, both autoclaves of the pair were vented at the same time into the vent exit manifold and condensers, causing a slowdown in the venting step, thus increasing the total venting time. Because of production demands and the high cost of industrial operations, it is desirable to increase the productivity of existing equipment.

It is the purpose of this invention to overcome these and other difliculties and to increase both the efficiency and the yield of the continuing twenty-four hour batch production of tetramethyl lead.

SUMMARY OF THE INVENTION In the operation of a group of tetramethyl lead autoclaves in which each autoclave is subjected to the steps of (1) charging with a catalyst, an inert diluent, a sodiumlead alloy and an autoclave lubricant; (2) charging with liquid methyl chloride; (3) heating to reaction conditions; (4) reacting the contents; (5) venting to a high pressure Patented Apr. 22, 1969 vent line; (6) venting to a low pressure vent line; and (7) discharging the contents: the improvements which comprise; (A) operating the group of autoclaves singly in sequence on a fixed time schedule; and (B) arranging the time schedule such that only one autoclave of the group vents at any one time to the low pressure line.

DESCRIPTION OF THE DRAWING The drawing is a schematic representation of a series of autoclaves connected to high and low pressure vent lines.

DETAILED DESCRIPTION OF THE INVENTION 7 Equipment employed in the operation Tetramethyl lead autoclaves, more commonly called methylation autoclaves, are well known in the art and require here only the briefest description. They are horizontal, cylindrical, steel pressure vessels jacketed for heating by steam or other means and for cooling by water, or other coolant. A means of agitation is provided which operates by essentially continually wiping or scraping the interior walls of the cylindrical vessel. The agitator shaft exits from both ends of the vessel through packing boxes serving as means of containing the pressurized autoclave contents. A means of rotating the shaft is provided at one end of the shaft and shaft rotation is usually at a speed of between about 4 and about 16 revolutions per minute. An autoclave is further provided with a scalable opening permitting charging of the catalyst, the diluent, the sodiumlead alloy and graphite (graphite serves as an internal lubricant for the scraping action of the agitator blades). Another opening, scalable by a valve, permits the charging of liquified methyl chloride under pressure to the autoclave. Still other valved openings permit and control venting to a reflux condenser and to the aforementioned venting systems. A means of discharging such an autoclave is provided by a scalable bottom-opening. Pressure gauges and/or pressure operated electrical switches are so installed that they are able to sense pressure and pressure changes inside the autoclave and that their electrical output signals can be used to operate electrical solenoids which, in turn, operate pneumatic valves in lines exiting from the autoclaves.

Condensation equipment for the volatile materials- The condensation equipment, as used in this invention and also in the older batch operation involving pairs of autoclaves, is also well known in the art. In the high pressure stage, a line from a manifold attached to the group of autoclaves runs to a brine cooled condenser which in turn feeds a rectification column. The column separates unreacted methyl chloride from the non-condensible byproducts of the methylation reaction and nitrogen and vents these non-condensibles to the atmosphere, while returning the purified methyl chloride to a storage tank for reuse.

The high pressure venting system has suflicient condensing capacity to simultaneously condense the ventings from more than one autoclave but the low pressure sy'stern, to be described next, is most effectively operated to condense the low pressure ventings from one autoclave at a time.

The low pressure condensation system also begins with a line running from a manifold attached to the group of autoclaves, said line feeds a brine-cooled condenser from which methyl chloride containing small amounts of toluene and tetramethyl lead are recovered and returned to a storage tank, usually the same tank as used for the methyl chloride from the high pressure system. Volatile materials not condensed in the low pressure system are compressed and passed into the brine cooled condenser of the high pressure condensation system. This is 3 done to afford a means, first, of collecting any residual methyl chloride vapor from the low pressure condenser and second to permit venting to the atmosphere of any non-condensibles from the low-pressure condenser; said non-condensibles being usually nitrogen, methane and ethane.

The preparation of tetramethyl lead in one individual autoclave In the ordinary batch procedure for preparing tetramethyl lead, the charging of an autoclave with anhydrous aluminum chloride, toluene, sodium-lead alloy and graphite is conducted with the autoclave at 60 C. or lower and using a positive pressure (about 0.1 kg./cm. gauge) of dry nitrogen to prevent exposing of the catalyst and the sodium-lead alloy to atmospheric moisture. The nitrogen is turned off after charging the above four materials and any excess internal pressure is vented to the atmosphere. After sealing the autoclave, liquid methyl chloride is charged into the autoclave and then a valve is closed to completely seal the autoclave from the methyl chloride source and to ready the autoclave for the next step, viz., the heating-up step. After this sealing and before heating and agitation are begun, internal autoclave pressure is between about and about kg./cm. gauge (gauge as used herein means pressure in excess of atmospheric), depending essentially on the autoclave temperature. Next, agitation, at eight revolutions per minute, and the heating up step are begun.

The heating-up step comprises admitting high pressure steam to the autoclave jacket until autoclave pressure reaches a point at which exothermic reaction has started and the heating with steam may be stopped. Continued heating in the reaction step is provided by exothermic reaction and is controlled by reflux rate of methyl chloride so as to keep internal autoclave pressure between about 16 and about 24.5 kg./cm. until reaction is substantially complete.

The heating-up step should be as rapid as permitted by the existing steam facilities and there should be no reflux cooling during this rapid heat-up step. Cooling by controlling methyl chloride reflux rate, is obtained either by manually or automatically varying the flow of coolant water to the methyl chloride reflux condenser, the latter by means of an interlocking time-pressure system, such as described in US. Patent 3,006,736. Such systems continuously vary the rate of flow of coolant water to the condenser. When the pressure rise in the autoclave exceeds a pre-set pattern in the time-pressure system, a pneumatic signal is transmitted to a mechanism controlling the coolant fiow supply, so that more coolant water flows through the condenser and, thereby, increases reflux rate of methyl chloride. The increased flow of cooled methyl chloride into the autoclave thus reduces the heating rate of the charge in the autoclave. Conversely, if pressure in the autoclave falls below the pattern of the timing mechanism, water flow is reduced, reducing methyl chloride reflux rate.

After the reaction period, the high and low pressure venting steps are begun. Cooling water is admitted to the methyl chloride reflux condenser and high pressure venting begun. When the pressure falls to about 6.3-7.7 kg./ cm. gauge, high pressure venting is ceased and low pressure venting commenced. During low pressure venting, cooling water is also admitted to the autoclave jacket.

The last step in the autoclave cycle comprises discharging the autoclave. Insofar as maximum usage of the autoclaves is concerned, once the discharge step is complete, an autoclave is ready for recharging.

The operation of the present invention As previously stated, the tetramethyl lead autoclaves were operated in pairs, whereas in the present invention they ar operated singly in sequence so that only one autoclave vents to the low pressure line at any one time.

After the reaction period, the high and low pressure venting operations are begun, as may best be illustrated by referring to the drawing. In a common venting line 2 leading from autoclave 1, valve 4 is opened to the high pressure venting line 6 by manually operating an electrical switch which activates a solenoid. The solenoid in turn activates pneumatically operated valve 4 which remains open until the pressure in autoclave 1 falls to between 6.3 and 7.7 kg./cm. gauge. At this point a pressure switch system operably senses that autoclave pressure has reached the above pressure range and actuates a solenoid which operates a pneumatic system to close valve 4 to the high pressure vent line 6 while a reciprocally operating solenoid, also activated by the pressure switch system, opens pneumatically operated valve 5 and begins the low pressure venting through line 3 into low pressure vent line 7. After about 10 minutes of low pressure venting, when gauge 13 shows about atmospheric pressure, a switch is thrown which so controls current to reverse the solenoid at valve 5 and allows valve 5 to close, effectively sealing the autoclave completely. The valves 5, 12, 1 8 are interconnected in such manner that only one may be opened at any one instant.

It is to be realized that the cut-off pressure which separates the high and low pressure venting steps can be raised or lowered according to the composition of the volatiles desired to be obtained from the ventings.

It will be appreciated that since only one autoclave vents to low pressure venting system at one time, the best fixed schedule time sequence is one based on the time taken to low presure vent the autoclave. This in turn is somewhat dependent upon the time required to charge the autoclave with methyl chloride. Thus valves 11 and 12 open on the same schedule respectively to high and low pressure venting through lines 9 and then line 10 and ultimately after the last autoclave 14 reaches the venting operation valves 17 and 18 open as in all previous parts of the sequence to vent this autoclave to the high and low pressure system first through line 15 and then through line 16.

The key to this rapid venting system lies in the separation of the two venting systems from each autoclave from each other and, more particularly, in the isolation of the low pressure venting system of each autoclave from that of the other autoclaves so that each autoclave can vent down to discharge pressure without bleeding pressure into another autoclave or overloading the venting condensers. This permits the low pressure venting condensation system to handle low pressure venting gases.

The interlocking pressure switch-solenoidpneumatic valve systems used to accomplish the separation and the isolation and the principles by which said systems are operably interconnected are known in the art, as exemplified by U.S.P. 2,863,737 and U.S.P. 3,006,736. Thus, the novelty of this invention does not lie in interconnected systems per se but rather in the use of these known systems to effect said separation and, particularly, said isolation.

The advantage of the preferred mode of operation of this invention and the comparison of that preferred mode with the older operation of a group of autoclaves in pairs is more fully illustrated by the following example from actual, commercial operations.

EXAMPLE 1 A 2300 liter methylation autoclave, as described above, is charged in an atmosphere of dry nitrogen in turn with about- Kg. Aluminum chloride catalyst 11.4 Toluene 86 Flaked sodium-lead alloy (NaPb) 1635 Graphite powder 13.5

The autoclave is then sealed and about 705 kg., that is about a two-fold stoichiometric excess of methyl chloride over the sodium-lead alloy, is forced into the autoclave under pressure. The methyl chloride valve is shut and the heat-up step and agitation are begun. Steam heating is maintained until internal pressure reaches about 17 kg./om. gauge at which point the reaction is exothermic and reaction is controlled by refluxing of excess methyl chloride. The heat-up step requires about 18 minutes. During the next step, an 86 minute reaction step, reaction rate is so controlled by refluxing methyl chloride as to keep internal pressure between about 17 and about 24.5 kg./cm. gauge until the 86 minutes has elapsed.

Cooling water is allowed to enter the condenser and high pressure venting is now begun and the autoclave is vented for 2 minutes to the high pressure condensation system. At the end of this 2 minute period, gauge pressure has dropped to between 6.3 and 7.7 kg./cm. in the autoclave and a pressure switch-solenoid pneumatic valve system closes the line from the autoclave to the high pressure system and opens the line from the autoclave to the low pressure condensation system, wherein venting is completed in about minutes down to about atmospheric pressure, and the autoclave is discharged to the steam still and immediately recharged and put back on cycle"as described above. During the low pressure venting stage cooling water flow is admitted to the autoclave jacket and to the condenser.

In the sequential operation of this invention, an autoclave immediately subsequent to the one above begins high pressure venting two minutes before the end of the low pressure venting of the above autoclave and starts its low pressure venting as the low pressure venting of the above autoclave is complete, ending its low pressure venting, therefore, 10 minutes after the above autoclave.

Sequential operation of autoclaves of Example 1 may best be illustrated by reference to FIGURE 1. Ten minutes after the sequence of steps for autoclave 1 is started, the sequence for autoclave 8 is started. Ten minutes later, the sequence is started for the next autoclave and so on.

At the end of the 86 minute reaction period, for autoclave 1, that is at 123 minutes after beginning of charging, autoclave 1 enters the high pressure venting stage for two minutes and at 125 minutes enters the low pressure venting stage for 10 minutes, concluding its low pressure venting at 135 minutes. At 133 minutes autoclave 8 has been put on high pressure vent for 2 minutes, starting low pressure venting at 135 minutes and ending low pressure venting at 145 minutes. These venting sequences are continued at 10 minute intervals as above until all 16 autoclaves have been so operated. (l3 autoclaves between autoclave 8 and autoclave 14 are represented by dashed line 21.) Thus the last autoclave No. 14 of FIG- URE l in the 16 is on schedule such that it enters high pressure venting at 273 minutes, counted from the start of charging autoclave 1, enters low pressure venting at 275 minutes and finishes low pressure venting at 285 minutes. Ventings are continuously condensed and noncondensibles are vented to the atmosphere from the high pressure condensation system 19 and low pressure condensation system 20 described previously.

In the older operation, pairs of autoclaves were charged as above and cycled simultaneously, essentially as in the immediately foregoing description. The second pair is started at a time sufiiciently later so that they are ready for venting when the first pair is finished. Likewise, the third pair is started at a similarly later time after the second pair and so on until all pairs are in operation. In the sequential operation, autoclaves are started on a 10- minute schedule. In the earlier operation, there was no set schedule for charging the autoclaves of one pair. Once the first autoclave was charged, charging of the second was begun regardless of how long was taken in charging the first.

The following table compares the actual commercial operation of running a group of 16 autoclaves of the capacity of the Example 1 autoclave in pairs and one-at-atime sequentially. Comparison is for a single autoclave, operating in one or the other of the above two modes of operation.

TAB LE Times in minutes l Based on the sodium-lead alloy.

The time employed for the high pressure venting can range from about 1 to about 5 minutes, and for the low pressure venting, from about 5 to about 12 minutes depending upon the venting components desired. Such times er'fiployed will depend upon raising or lowering the cutoff pressure between the high and low pressure steps.

In the preferred method of operation, 16 autoclaves of about 2300 liters capacity each are operated on a sequence of 10-minute intervals, thus permitting a maximum of 144 complete autoclave cycles per 24-hour day (1440 minutes). It is noteworthy that the preferred sequential operation of one autoclave at a time has attained a 91% efficiency, i.e., 131 cycles per day while with the older, pair operation, at best, only 88% efiiciency, i.e., 127 complete cycles per 24 hour day could be attained. The loss of one autoclave still allows the attainment of the 91% efiiciency of the entire group, because the'overall cycle time can be reduced to minutes at a sacrifice of 10 minutes of reaction time. The high efficiency offsets loss of yield due to the decreased reaction time. In contrast, loss of one autoclave in the paired operation reduces the efficiency by A It will be appreciated, from an examination of these results, that sequential operation, the method of this invention, produces significantly better results both in number of charges per day and in yield. Yield increase is believed due to the increased reaction time afforded by placing the sequence on the time schedule of this invention.

It is to be realized that the time figures provided in the foregoing discussion will vary as capacity of the equipment employed varies.

The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for obvious modifications will occur to those skilled in the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as fole lows:

1. In the process of operating a group of tetramethyl lead autoclaves in which each autoclave is subjected to the steps of (1) charging with a catalyst, an inert diluent, a sodium-lead alloy and an autoclave lubricant; (2) charging with liquid methyl chloride; (3) heating to reaction conditions; (4) reacting the contents; (5) venting to a high pressure vent line; (6) venting to a low pres sure vent line; and (7) discharging the contents: the improvements which comprise:

(A) operating the group of autoclaves singly in sequence on a fixed time schedule; and

(B) arranging the time schedule such that only one 7 autoclave of the group vents at any one time to the low pressure vent line.

2. The process improvement according to claim 1 wherein the time schedule is such that the time for venting step (6) is at most no shorter than for charging step (2).

3. The process improvement according to claim 1 wherein the pressure cut-off between steps (5) and (6) is between about 6.3 and about 7.7 kg./cm. gauge.

4. The process improvement according to claim 1 wherein the time taken for step (5) is between about 1 and about 5 minutes, and the time taken for step (6) is between about 5 and about 12 minutes.

5. The process improvement according to claim 1 wherein each autoclave has a capacity of about 2300 liters and the amount of the reactants charged in steps (1) and (2) is about 11.4 kg. of aluminum chloride catalyst, about 86 kg. of toluene diluent, about 1635 kg. of sodium-lead alloy, about 13.5 kg. of graphite lubricant and about 705 kg. of methyl chloride.

References Cited UNITED STATES PATENTS 5/1939 Schuman 23-260 12/1958 Green.

7/1959 Erickson 23-290 XR 10/1961 Green.

8/1962 Jarvie et al. 260-437 8/1962 Cook et al. 260-437 9/1962 Kirkpatrick et al. 23-260 XR 1/ 1963 Tullio 260-437 5/1963 Eastman et al. 23-260 XR U.S. Cl. X.R. 

